As you said, you want positional cues through headphones. That means dolby headphone (is there a DTS equivalent to this? I don't know), which is basically an effects processor that puts positonal cues into a 2 channel output.
It's possible, on my Xonar D2X, to output the dolby headphone treated sound via either analog (through the jacks on the soundcard itself) or SPDIF digital output to an external DAC.
I don't think you need a DAC - the DACs on a good sound card are really rather good (the DAC chips on my D2X are better than most sub $1000 DACs), and unless you are planning on spending $2000 on a pair of headphones, I really doubt you'd get any benefit from an external DAC, provided you are using a decent soundcard.
You can, as someone else said, use just a DAC and use the motherboard digital output to feed it, but I expect you will find that the motherboard sound chip can't do dolby headphone, probably because of the licensing cost. Whether or not you can get DACs that do dolby headphone, I don't know.
If you did use just a DAC, then you would need a headphone amp. Using a sound card, the headphone amp is built into the sound card. Sure, you could use the line-level outputs from the sound card (assuming you buy a card that has line level outputs) and feed them into an external headphone amplifier, but again - unless you buy very high-end headphones, you'll find that the amp on a top of the range soundcard is really very good - good enough that you don't need anything else with "good" headphones.
So the best solution for you would probably be a good quality sound card, and some nice headphones, I have a Xonar D2X and Sennheiser PC360 G4ME headphones, and for gaming they are absolutely outstanding. Music - meh, they are OK, in fact they are the best headphones I've ever heard, but I have pretty good stereo speakers and compared to proper speakers they aren't so great, but I expect that even applies to really expensive headphones just because of the restrictions of size and weight.
EDIT: It occured to me to mention head-fi.org to you, then I just noticed this thread: http://www.head-fi.org/t/633226/best-way-to-get-dolby-headphone-dac-amp-that-accepts-dolby-digital-live-also-usb-dac-amp-with-dolby-headphone-support
It seems that Dolby Headphone is much better than the creative alternative, but it seems like something called CMSS-3D (which I've never heard) did the same as Dolby Headphone, but is not in the newer creative cards. I'd tread carefully, and do your research thoroughly.
I also note that someone brought up the important point in that head-fi.org thread, that a DAC won't give any gaming support (won't support the EAX/DirectSound stuff that games do). That's pretty important, so I'd definitely veer away from a DAC if I were you.
The Xonars don't do EAX in hardware (I think because of a licensing issue) but they do emulate it in the driver in such a way that it makes no difference - they basically fully support it, and on modern hardware, the processing load of it being done by the CPU rather than natively on the sound card is totally insignificant.
I'd look at the Xonar cards, because last I looked, Creatives drivers were appalling.
Get thee to head-fi.orgEdited by BorisTheSpider - 11/7/12 at 1:17pm